Bloomington to survey Bush Lake for weeds

Bloomington's Bush Lake will not be treated for invasive weeds next spring, but the city will conduct two surveys to figure out how bad the problem is.

Those two tests are necessary precursors to any chemical treatment for Eurasian water milfoil and curlyleaf pondweed, two non-native weeds that in recent years have choked boat motors and clung to canoe paddles on the city's premier recreational lake.

"Everybody's in agreement that Bush Lake is a unique resource in Bloomington that people want to maintain and preserve," said Scott Anderson, city senior civil engineer who works with water resources. "There's a desire that if something can be done and [the cost] is reasonable, that's something that should be pursued.

"But there are a number of water resources in the city, and we can't do everything everywhere."

Since at least 1990, the lake has been infested with milfoil, which forms dense mats in water less than 15 feet deep.

Curlyleaf pondweed is another unwelcome lake resident, though it is not as troublesome to swimmers and fishermen.

This fall, the Bush Lake chapter of the Izaak Walton League asked the City Council to seek a state grant to help pay for treatments to fight the weeds, which they said are threatening to ruin swimming and fishing on the lake. Council members were sympathetic and agreed that the lake is a precious resource, but they wanted to know how much treatment would cost and how effective it would be.

Treating large portions of the lake for aesthetic and recreational reasons would be a policy departure for Bloomington. Historically the city has treated smaller lakes and ponds to improve water quality. But Bush Lake has very good water quality now. The only area the city has treated with chemicals to kill weeds is near the swimming beach. That's been done annually at least since 2003.

While Izaak Walton League members had hoped that the city would apply for a state Department of Natural Resources (DNR) grant to treat the weeds next year, Anderson said that first the city has to figure out what the lake's conditions are. The tests the city will do are required by the DNR before a grant application, Anderson said.

Two weeks ago, 30 sediment samples were taken from the lake's bottom in areas shallower than 15 feet, where milfoil and curlyleaf pondweed flourish. The University of Minnesota will test the samples for phosphorus and nitrogen levels, to see how conducive the sediment is to invasive weed growth. That test will cost the city about $4,200.

The second test, a vegetation survey, will be done in the spring to get more exact measures of where milfoil and curlyleaf pondweed are growing. Anderson said that is expected to cost $2,000 to $4,000.

If the city decided to go ahead and treat the lake, the city estimates that the entire project may cost $175,000 to $189,000 over three years.

While chemicals can significantly knock back milfoil, infested lakes often have to be treated again after five or six years.

"You can't just go out there one time and have success with this kind of stuff," Anderson said. Monitoring has to be ongoing to gauge the effect on water quality and native plants, he said.

If the city went ahead with a treatment program, the bulk of the expense would fall on Bloomington. The league has pledged $1,000 toward that effort. DNR grants generally run about $10,000 a year.

Gregg Thompson, president of the Bush Lake chapter of the league, said league members "would have loved to see treatment starting next year. But this is the next best thing to that."

He said he was cheered that City Council members seem supportive of efforts to improve the lake, and he said he hoped other organizations would come forward to contribute to the cost if chemical treatment eventually happens.